Three phase, three wire or four wire electrical systems require detectors for detecting unbalanced phase currents. It is preferable that the detectors use equipment already available in such systems. The only available equipment suitable for this purpose includes an arrangement of three current transformers, one for each phase current, used to detect overcurrent conditions in the system.
As will be understood by those skilled in the art, in order to detect overcurrent conditions the outputs of the current transformers are full-wave rectified and applied through a burden resistor. Because of the rectification, the voltage wave-form across the burden resistor is non-sinusoidal and, as a result, conventional means for detecting unbalanced currents, such as negative sequence currents requiring phase shifting networks, cannot be used.
The present invention makes use of the harmonic content of a rectified ripple voltage developed across the burden resistor to detect unbalanced current conditions. In this connection, it is noted that the current transformers are essentially current sources. The burden resistor accomplishes a current vector addition which provides a ripple voltage across the resistor at a predetermined frequency (2400 Hz, for example) when the nominal (400 Hz, for example) phase currents are balanced. An unbalanced, ungrounded system load provides a component at a predetermined frequency (800 Hz, for example) which is proportional to the degree of unbalance. An unbalanced load whose neutral is grounded provides a component at another predetermined frequency (400 Hz, for example) which is proportional to the degree of unbalance. An appropriate filter (low-pass) can be used to pass the 400/800 Hz components while rejecting the 2400 Hz component, and hence a dependable indication of current unbalance is provided.